1. The problem statement, all variables and given/known data Trying to find the number of diffraction maxima and the width of the whole pattern. [itex]\lambda = 500nm[/itex] [itex] b = 2.8μm[/itex] (Slit Width) [itex] f = 100mm[/itex] [itex] r = 25mm[/itex] [itex] L = 25mm[/itex] (Slit to Lens) 2. Relevant equations I have the worked out the irradiance formula as: [tex] I(\theta)=I_0 sin^2(\beta)/(\beta^2)[/tex] The conditions for minima [tex] b sin\theta = m\lambda [/tex] Angular width of central fringe [tex] \Delta\theta \approx 2\lambda/b[/tex] Width of central fringe [tex] W \approx 2L\lambda/b [/tex] 3. The attempt at a solution As said I worked out the irradiance formula, and I think to find the width of the whole pattern you would do: [itex] (Fringe Width \times 2) + 1 [/itex] As then it accounts for the dark fringes as well, plus the one extra for the central. As for working out the number of maxima, I would guess it would involve the condition for a minimum as it contains the order of diffraction. What really throws me off though, is the lens, I'm not sure what that would do for the calculations.